Lithium Aluminium Hydride and Metallic Iron: A Powerful Team in Alkene and Arene Hydrogenation Catalysis

Alkenes that normally do not react with LiAlH4 (3-hexene, cyclohexene, 1-Me-cyclohexene), can be reduced to the corresponding alkanes by a mixture of LiAlH4 and Fe-0 (the iron was activated by Metal-Vapour-Synthesis). This alkene-to-alkane conversion with a stoichiometric quantity of LiAlH4/Fe-0 does not need quenching with water or acids, implying that both H's originate from LiAlH4. The LiAlH4/Fe-0 combination is also a remarkably potent cooperative catalyst for hydrogenation of multi-substituted alkenes and benzene or toluene. An induction period of circa two hours and the minimally required temperature of 120 degrees C, suggests that the actual catalyst is a combination of Fe-0 and the decomposition product of LiAlH4 (LiH and Al-0). A thermally pre-activated LiAlH4/Fe-0 catalyst did not need an induction time and is also active at room temperature and 1 bar H-2. A combination of AliBu(3) and Fe-0 is an even more active hydrogenation catalyst. Without pre-activation, tetra-substituted alkenes like Me2C=CMe2 and toluene could be fully hydrogenated.

Automated summary

Alkenes that are normally unreactive with LiAlH4 can be reduced to alkanes by a mixture of LiAlH4 and Fe-0. This reaction does not require the use of water or acids and the H atoms for the reduction come from LiAlH4. The LiAlH4/Fe-0 combination is also an effective catalyst for hydrogenation of multi-substituted alkenes and benzene/toluene.